The present invention relates to an automatic equalizer used for a digital radio communication system, especially to an automatic equalizer comprising an adaptive matched filter and a decision feedback equalizer to remove an intersymbol interference due to radio channel fading and a method for controlling tap coefficients used therein.
A conventional digital radio communication system has been so proposed to combine an adaptive matched filter (AMF) and a decision feedback equalizer (DFE) at a receiver side to compensate degradation of the channel quality due to frequency selective fading environment generated on a propagation path (See Paper No.B-929, Spring National Convention of Electronic Information Communication Society, 1989).
Supposing that an amplitude ratio of a reflected (delayed) wave to a direct (preceding) wave is .rho., the DFE unity capable of equalizing two wave interference under fading functions in equalizing the intersymbol interference completely in case of .rho.&lt;1. While in case of .rho.&gt;1, its performance is deteriorated, implying that the equalization characteristic is degraded when the amplitude of the delayed wave becomes larger than that of the preceding wave (See Chapter 6 of "Digital radio communication" written by Muroya and Yamamoto, published by Sangyo Tosho).
In order to improve the equalization characteristic in .rho.&gt;1, it has been proposed to place the AMF ahead of the DFE.
FIG. 4 is a first block diagram of a conventional automatic equalizer. A reference numeral 1 is an A/D converter. Reference numerals 2 and 3 are delay circuits which delay a symbol space by T. Reference numerals 4 to 6 are multipliers. Reference numeral 7 is an adder, 8 to 10 are correlators, 11 to 13 are integrators, 101 is a demodulator, 201 is a transversal filter, and 301 is a decision feedback equalizer (DFE).
The adaptive matched filter of a T space 3 tap type is composed of the transversal filter 201, the correlators 8 to 10, and the integrators 11 to 13.
The operation of the automatic equalizer of this type is hereinafter described.
The demodulator 101 demodulates a received signal, and a demodulated signal is sample-quantized by the A/D converter 1. The sample-quantized digital signal string output from the A/D converter 1 is input to the transversal filter 201, then delayed by the delay circuits 2 and 3 sequentially. An input signals of the transversal filter 201 and output signals of the delay circuits 2 and 3 are respectively input to the multipliers 4 to 6 where they are multiplied by tap coefficients A.sub.-1, A.sub.0, and A.sub.+1, respectively. Outputs of the multipliers 4 to 6 are added through the adder 7. The output of the adder 7 is input to the DFE 301 where the intersymbol interference is equalized and decided.
A decision signal 104 output from the DFE 301 is input to the correlators 8 to 10 where the decision signal 104, the input signal of the transversal filter 201 and output signals of the delay circuits 2 and 3 are correlated. The respective mean values of outputs of the correlators 8 to 10 are obtained through the integrators 11 to 13 to define new tap coefficients A.sub.-1, A.sub.0, and A.sub.+1 of the transversal filter 201.
Convolution of those tap coefficients and received signals provides outputs of the adaptive matched filter (AMF) corresponding to the impulse response on the propagation path. FIG. 6 shows waveforms of the output response in case of .rho.&gt;1. FIG. 7 shows waveforms of the output response in case of .rho.&lt;1.
The adaptive matched filter (AMF) serves to disperse the interference wave component due to frequency selective fading generated on the propagation path ahead and behind the main wave component, respectively as shown by FIGS. 6 and 7.
In case of .rho.&gt;1, i.e., the amplitude of the delayed wave is larger than that of the preceding wave, the AMF enables the preceding wave as the interference component to be dispersed ahead and behind the main wave. With this mechanism, the interference wave component preceding the main wave that is difficult to be equalized by the DFE located behind the AMF is converted into that behind the main wave, thus the equalization characteristic of this case can be improved.
In case of .rho.&lt;1, however, the construction of the first example of the conventional automatic equalizer generates the interference wave component preceding the main wave as FIG. 7 shows. Accordingly the equalization characteristic is degraded compared with the construction using the DFE unity only.
Aiming at overcoming the aforementioned degradation, another type of the equalizer has been proposed in Japanese Patent Laid-Open No.77106(1992). It is so constructed to operate the DFE unity only by suspending the AMF in case of .rho.&lt;1. While in case of .rho.&gt;1, the AMF and DFE are operated together. The operation of the AMF is suspended by fixing the tap coefficients of FIG. 5 to initial values A.sub.0 =1, A.sub.-1 =A.sub.+1 =0 (The DFE unit only). This mechanism is realized by adding a control circuit for controlling those tap coefficients to the construction shown by FIG. 4.
FIG. 5 is a block diagram of a second conventional automatic equalizer. The same construction items as those shown by FIG. 4 have the same reference numerals, thus omitting their explanations.
Referring to FIG. 5, reference numerals 30 and 31 are multipliers, 32 is a control signal generator, and 402 is a control circuit.
The control signal generator 32 monitors outputs of the integrators 11 to 13 which represent impulse responses on the propagation path to determine the status thereof.
If the propagation path status is determined as .rho.&gt;1, the control signal generator 32 outputs 1 to multipliers 30 and 31, respectively to operate the AMF and DFE in the same manner as described in the first example of a conventional automatic equalizer. While if determined as .rho.&lt;1, it gradually decreases the value output to the multipliers 30 and 31 to 0 so as to operate the DFE unity only.
In this case, the AMF generates outputs without diverging the interference wave as shown in FIGS. 8 and 9.
Adding the control circuit 402 provides higher equalization characteristic with the automatic equalizer compared with the first example on the propagation path status in case of both .rho.&gt;1 and .rho.&lt;1.
When using this equalizer as the second example, in case the propagation path is determined as .rho.&lt;1 and the interference wave component becomes excessively large, it no longer performs its equalization characteristic appropriately, which causes output signals to be dispersed. In this case, the closer .rho. approaches the value 1, the more difficult it becomes to perform pull-in operation, i.e., converging the location of output signals of the equalizer.
If the propagation path is determined as .rho.&lt;1, the AMF is reset (suspended) and DFE unity is only operated. So the interference wave component is not dispersed by the AMF, requiring some DFE taps to have substantially a great value.
When the propagation path status is in the vicinity of .rho.=1, it may be further difficult to perform pull-in operation due to additional AMF reset control.
In the first example of the prior art of FIG. 4, the AMF disperses the interference wave component to require no great value of tap coefficient for the respective DFE taps even if the communication path status is in the vicinity of .rho.=1, resulting in easy pull-in operation.
In this case, however, as aforementioned, the equalization characteristic is degraded during pull-in status.